Having trouble getting a solid color

[Mike Lundy]

Hi. I'm an EE student, and our class is currently working on a lab that
involves VGA output. (This question does in fact relate to homework, but
helping me out here is not enabling me to cheat- you'll just have to
take my word for that. Just wanted to get that out of the way first).

Anyway. VGA output. We're using a Xilinx Spartan-3[1] board. I'm just
learning how to output vga, but I seem to be having a problem getting a
solid color to output- if I output a stripe taller than a certain
length, the signal begins to fade, eventually to black. At the moment,
I'm outputting a rectangle framed by the pixels (x,y) (0,0 @ UL) :
(0,200) -> (640,300). I've adjusted for the porch ranges.
~60Hz Vsync, ~31.2KHz Hsync.

The band is a roughly linear gradient over the vertical range. I'm not
sure what I'm supposed to do to get it to output one solid color. We're
doing this in VHDL, but I'm really looking for understanding, not code
(unless that code aids understanding )

Thanks bunches,
Mike Lundy

1) Datasheet here, if you care:
http://www.digilentinc.com/Data/Products/S3BOARD/S3BOARD-rm.pdf
Basically, we get 1 bit each for RGB, plus Hs/Vs.

 

[Bob Myers]

Anyway. VGA output. We're using a Xilinx Spartan-3[1] board. I'm just
learning how to output vga, but I seem to be having a problem getting a
solid color to output- if I output a stripe taller than a certain
length, the signal begins to fade, eventually to black. At the moment,
I'm outputting a rectangle framed by the pixels (x,y) (0,0 @ UL) :
(0,200) -> (640,300). I've adjusted for the porch ranges.
~60Hz Vsync, ~31.2KHz Hsync.

When you say the signal "fades to black," are you talking
about what it looks like on the display - and if so, what
is the display in question? - or are you monitoring the
signal on a scope and you're seeing a loss of amplitiude?

Bob M.

 

[Mike Lundy]

Anyway. VGA output. We're using a Xilinx Spartan-3[1] board. I'm just
learning how to output vga, but I seem to be having a problem getting a
solid color to output- if I output a stripe taller than a certain
length, the signal begins to fade, eventually to black. At the moment,
I'm outputting a rectangle framed by the pixels (x,y) (0,0 @ UL) :
(0,200) -> (640,300). I've adjusted for the porch ranges.
~60Hz Vsync, ~31.2KHz Hsync.

When you say the signal "fades to black," are you talking
about what it looks like on the display - and if so, what
is the display in question? - or are you monitoring the
signal on a scope and you're seeing a loss of amplitiude?

Yeah, what it looks like on the display (3 is solid color, 0 is black):
|-------------------|
|0000000000000000000|
|3333333333333333333|
|2222222222222222222|
|1111111111111111111|
|0000000000000000000|
|0000000000000000000|
|-------------------|

It's a fairly run-of-the-mill CRT, from maybe two years ago. I'd give you
the model & such but I'm off campus right now. Oh! Also, one of my
classmates tried it with an LCD from this year, with exactly the same
effect. I'd like to monitor it on a scope to be sure, but another class has
grabbed all the probes small enough to do the job properly. >:|